In recent years, to achieve energy saving, the development of highly efficient boilers has been advanced. For example, an ultra supercritical pressure boiler uses higher temperature and pressure of steam than those in a conventional boiler to enhance the energy efficiency. Also, a boiler using wastes or biomass as a fuel other than fossil fuels has been developed. Further, there has been advanced the development of an electric power plant boiler utilizing solar heat has been developed. In particular, a solar thermal power plant boiler has attracted attention from the viewpoints of energy saving and environmental preservation. As a steel material of heat exchangers and the like for these boilers, a heat resistant ferritic steel may be used. The boiler steam temperature is high, and reaches a temperature close to 600° C. in some cases. The heat resistant ferritic steel used in such an application is required to have excellent photoselective absorptivity.
The photoselective absorptivity is a property such that absorptivity changes in different wavelength regions. The term of “excellent photoselective absorptivity” means that, for light (electromagnetic wave) of visual to near-infrared region (wavelength: 0.3 to 1 μm, hereinafter referred to as “low wavelength side”), the absorptivity is high, and for light (electromagnetic wave) of medium- to far-infrared region (wavelength: 2.5 to 25 μm, hereinafter referred to as “high wavelength side”), the radioactivity is low. In other words, the photoselective absorptivity means that the reflectance of light on the low wavelength side is low, and the reflectance of light on the high wavelength side is high.
To attain excellent photoselective absorptivity, various methods have been proposed so far. JP52-126434A (Patent Document 1) and JP58-195746A (Patent Document 2) disclose methods in which the photoselective absorptivity is enhanced by forming an organic coated film on the surface of steel material. The paint disclosed in Patent Document I consists of semiconductor particles having an energy band width of 0.4 to 1.5 eV, a polyvinyl butyral organic binder, and a solvent. The paint for photoselective absorbing film disclosed in Patent Document 2 contains carboxylic acid amide copolymer, oxides, and solvent-based paint.
JP53-75132A (Patent Document 3), JP60-57157A (Patent Document 4), and JP62-182553A (Patent Document 5) disclose methods in which, to attain the photoselective absorptivity, triiron tetraoxide (Fe3O4: magnetite) is formed on the surface of steel by chemical treatment or the like. Specifically, in Patent Document 3, a selective absorbing film consisting of magnetite is formed by immersing a base material consisting mainly of iron in a high-temperature alkaline solution. In Patent Document 4, a selective absorbing film consisting of magnetite is formed by electrooxidizing a base material consisting mainly of iron in an acidic solution. In Patent Document 5, a selective absorbing film consisting of magnetite is formed by electrooxidizing a base material consisting mainly of iron in an acidic solution after the surface of base material has been iron-plated.
JP55-77667A (Patent Document 6) discloses a method in which an oxide film consisting mainly of Fe that has a film thickness of 500 to 2000 angstroms and contains 11.00 to 26.00 wt % of Cr is formed by a chemical treatment method or the like method, and the surface of oxide film is mirror-polished. Patent Document 6 describes that the photoselective absorptivity is enhanced by this method.
JP7-325212A (Patent Document 7) discloses a method in which a film consisting of iron oxide is formed on the surface of steel by spraying. Patent Document 7 describes that the photoselective absorptivity is enhanced by this method.